1. Field of the Invention
The present invention relates to the creation, replacement, restoration or reinforcement of concrete structure connections.
2. Description of Related Art
Precast, prestressed concrete parking garages have become one of the most popular methods for parking garage construction. This construction consists of fabrication of discrete precast concrete elements or members that are transported to the construction site, lifted into position and connected together.
As shown in a prior art drawing of FIGS. 1 and 2, precast concrete structures typically consist of “tee” or “double tee” beams 26, girders 34, columns 32 and lateral bracing members 30. A lateral bracing member 30 may be a shear wall, brace or frame. The tee/double tee beams are connected together to form the floor/deck. The deck supports gravity loads from vehicles and may be formed from one or more diaphragms. Diaphragms are, in effect, large horizontal beams that transmit lateral loads from seismic and wind events to points of lateral support, typically lateral bracing members 30. The diaphragm may be composed of one or more rows of precast concrete members 26. To form the deck the tee beams are connected to one another with two types of connections, shear connections and chord connections. Tension, compression, vertical shear and horizontal shear are each addressed with connections to resist the respective force or load. Shear connections 56 provide mutual support to gravity (wheel) loads while also transmitting shear within the plane of the deck. Chord connections 58 provide mutual support to gravity (wheel) loads and form chords that restrain tension forces due to in-plane bending of the diaphragm in the manner similar to that in which reinforcement steel restrains tension in a concrete beam under bending. FIG. 2 shows welded chords 45 extending from one end of the diaphragm to the opposite end.
An example of the precast member 26 is a double tee beam shown in FIG. 3 and includes a platform having a length L and a width W. The platform consists of a center panel 40 and flanges 24 extending outward from the center panel. The double tee beam 26′ includes a pair of beam webs 12 extending below the platform, the beam web having beam web ends 44 and beam web sides 46 extending between the beam web ends 44. The flange 24 includes flange ends 38 substantially parallel to the beam web 12.
Chord connections are typically used in precast concrete structures and are discussed in U.S. patent application Ser. No. 12/569,246 by the inventor of the present invention. U.S. patent application Ser. No. 12/569,246 is hereby incorporated by reference. Chord connections 58 are shown in FIGS. 4A-4C while shear connections 56 may encompass the connections shown in FIGS. 4A-4C as well as FIG. 4D and provide stability from shear forces. Both shear connections and chord connections are typically achieved by placing a loose erection plate 22′ or erection bar 22 between adjacent flanges 24 and welding them to steel plates 20, 20′ embedded within the precast members. Embedded steel plates 20, 20′ are installed during fabrication of the concrete member prior to placement of the concrete in the formwork. Anchorage of the plate within the concrete is typically achieved by welding the plate to a steel bar, stud or rebar 28 prior to fabrication. The steel plates are positioned within the end of the flange 24 such that the face of each plate is exposed on the end of the flange. Once the tee beam is placed into position during fabrication of the concrete structure, the gap between embedded plates 20 are bridged with a loose erection bar 22′ or dowel 22 and welded together. Once welded, this creates a continuous steel chord within the edge of the diaphragm/deck. The chord connections 58 have an upper cavity 90 which may be filled with a sealant and/or grout. Alternatively or in combination, a layer of concrete may be spread across the top of the flange 24 and flange connection. The chord connections 58 form chords 45 which are embedded in the platform of the diaphragm. The steel bar 28, the steel plate 20 and the erection bar or plate 22 make up the chord 45 and typically extends through all of the platforms in the diaphragm.
The shear connection shown in the top plan view of FIG. 4D is a proprietary Vector Connector 41 consisting of a bent plate 25b with wings 25a embedded in the flange 24, and although it is not attached to rebar, the wings 25a provide sufficient anchorage to act as a shear connection. The bent plate 25b is welded to the bent plate of the adjacent Vector Connector 41 using an erection plate 22′. The Vector Connector is not sufficient for tension forces and usually supplements the welded chord connections. The Vector Connections are usually spaced between the chord connections 58 which are along or near the outer edges of the diaphragm.
The welded chords 45 shown in FIG. 2 typically include the chord connections 37 and the embedded steel rods 28. After welding the chord connections during fabrication and erection of the concrete structure, the chord connections and the steel rod form a continuous welded chord. The chord connections 58 mostly determine the integrity of the welded continuous chord.
The existing methods of construction produce numerous problems and failures, generally due to poor design, poor fabrication in the shop, poor construction in the field, and in service deterioration. Welds of the type required require special procedures and specially trained welders and welding code provisions do not allow welds to be used in the manner in which they are loaded in this connection. Quality control/inspection of welding in the shop during fabrication and in the field during erection is essential to the performance of this connection. This quality control is difficult and costly to perform during fabrication and erection and in service.
When the embedded plates are welded during erection of the concrete structure, the heat of welding causes expansion of the plates causing cracks to form in the concrete. Road salts deposited at or near the connection then cause deterioration due to corrosion, especially in cold climates where high concentrations of salt is used to reduce or prevent ice formation. Additionally, the cracks in the concrete allow water to enter, creating further concrete deterioration when the water does freeze. To slow the progress of corrosion at these connections, stainless steel is often used, greatly increasing the cost of production both in the material cost and the construction labor since stainless steel is more costly and more difficult to weld. The stainless steel requires special welder qualifications and expands more than carbon steel, increasing cracks due to expansion. In addition, the steel plates 20 are typically stainless steel and are typically welded to carbon steel rebar, causing a galvanic reaction that accelerates deterioration of the connection.
The connections located within the joint between beams disrupt sealant disposed in the connection joint causing leaks to occur at the connection, accelerating their deterioration and creating leaks within the garage.
The Vector Connections 41 have quickly become the norm for creating shear type connections in precast garages. This has had unintended consequences with regard to chord connections. Previously, shear connections were erected the same way as chord connections and aided in carrying tensile stress within the deck due to restraint to thermal volumetric changes. The Vector Connections are very flexible and provide little to no restraint to tension forces. This previously shared load is instead transmitted to the chord connections, increasing the stress on these components and causing them to fracture.
The diaphragm shown in FIG. 5 consists of multiple double tee beams 26. Seismic or wind loading indicated by the arrows forces the diaphragm to arc or bend laterally. These forces create tension between the concrete members 26 in one part of the diaphragm and compression in between the concrete members in another part of the diaphragm. These forces also create shear between the adjacent concrete members 26. As these forces increase, the diaphragm continues to bend laterally.
Since welded connections are inflexible, the flexibility restraint between points of lateral support creates great stress due to volumetric change from thermal variations, causing connections to fracture. There is a need for a shear connector which provides stability of the concrete structure from shear forces during seismic, wheel and wind loading while providing a connection which allows a small amount of flexibility due to tension and compression forces due to volumetric change from thermal variations, preventing damage to the concrete structure.